Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, engine braking, and exhaust gas recirculation (EGR). During positive power, one or more intake valves may be opened to admit fuel and air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves may also be opened during positive power at various times to recirculate gases for improved emissions.
Engine valve actuation also may be used to produce engine braking and exhaust gas recirculation when the engine is not being used to produce positive power. During engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle.
Engine valve(s) may be actuated to produce compression-release braking and/or bleeder braking. The operation of a compression-release type engine brake, or retarder, is well known. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston approaches the top dead center (TDC), at least one exhaust valve is opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is incorporated herein by reference.
The operation of a bleeder type engine brake has also long been known. During engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the former does not have exhaust valve lift during most of the intake stroke. An example of a system and method utilizing a bleeder type engine brake is provided by the disclosure of Assignee's U.S. Pat. No. 6,594,996 (Jul. 22, 2003), a copy of which is incorporated herein by reference.
The basic principles of exhaust gas recirculation (EGR) are also well known. After a properly operating engine has performed work on the combination of fuel and inlet air in its combustion chamber, the engine exhausts the remaining gas from the engine cylinder. An EGR system allows a portion of these exhaust gases to flow back into the engine cylinder. This recirculation of gases into the engine cylinder may be used during positive power operation, and/or during engine braking cycles to provide significant benefits.
During positive power operation, an EGR system is primarily used to improve engine emissions. During engine positive power, one or more intake valves may be opened to admit fuel and air from the atmosphere, which contains the oxygen required to burn the fuel in the cylinder. The air, however, also contains a large quantity of nitrogen. The high temperature found within the engine cylinder causes the nitrogen to react with any unused oxygen and form nitrogen oxides (NOx). Nitrogen oxides are one of the main pollutants emitted by diesel engines. The recirculated gases provided by an EGR system have already been used by the engine and contain only a small amount of oxygen. By mixing these gases with fresh air, the amount of oxygen entering the engine may be reduced and fewer nitrogen oxides may be formed. In addition, the recirculated gases may have the effect of lowering the combustion temperature in the engine cylinder below the point at which nitrogen combines with oxygen to form NOx. As a result, EGR systems may work to reduce the amount of NOx produced and to improve engine emissions. Current environmental standards for diesel engines, as well as proposed regulations, in the United States and other countries indicate that the need for improved emissions will only become more important in the future.
An EGR system may also be used to optimize retarding power during engine braking operation. As discussed above, during engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. By controlling the pressure and temperature in the engine using EGR, the level of braking may be optimized at various operating conditions.
Generally, there are two types of EGR systems, internal and external. Many conventional EGR systems are external systems, which recirculate the gases from the exhaust manifold to the intake port through external piping. Some external EGR systems require several additional components, such as, external piping, bypass lines, and related cooling mechanisms, in order for the system to operate properly. These additional components may significantly increase the cost of the vehicle, and may increase the space required for the system, creating packaging and manufacturing concerns. Many conventional internal EGR systems provide EGR by taking exhaust gas into the combustion chamber through an open exhaust valve during the intake stroke, or through an open intake valve during the exhaust stroke.
In many internal combustion engines, the valve train motion for producing an engine valve event in a first cylinder may be used to produce an internal exhaust gas recirculation event in a second cylinder. For example, the motion provided by the main intake valve event lobe on the cam of the first cylinder may be used to produce an EGR event in the second cylinder. Some “cross-actuated” EGR systems, however, may be difficult to package within the constraints of a vehicle.
An advantage of embodiments of the present invention is that engine intake and exhaust valves may be opened and closed by fixed profile cams, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. Using, for example, an EGR lobe ground into the cam of the same engine cylinder where the valve event is to occur may significantly reduce packaging difficulties. Other benefits such as increased performance, improved fuel economy, lower emissions, and better vehicle drivability may be obtained by optimizing the engine valve timing and lift.
The use of fixed profile cams, however, can make it difficult to adjust the timings and/or amounts of engine valve lift to optimize them for various engine operating conditions. One method of adjusting valve timing and lift, given a fixed cam profile, has been to provide valve actuation that incorporates a “lost motion” system in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, and/or other linkage assembly. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
The systems and methods of the present invention may be particularly useful in engines requiring valve actuation for positive power, engine braking valve events and/or EGR/BGR valve events. The systems and methods of various embodiments of the present invention may provide a lower cost, production viable variable valve actuation system that are more easily packaged than other engine EGR systems. Additional advantages of embodiments of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.